The invention relates to a fluid friction coupling, especially for a cooling fan of a fluid-cooled internal combustion engine.
A fluid friction coupling for a cooling fan of an internal combustion engine is known from European Patent Application No. 9,959, in which the internal combustion engine drives a rotor of substantially disc form seated on a drive shaft. A housing carrying the fan blades is rotatably mounted on the drive shaft. The housing contains a reservoir for shear fluid of high viscosity which is separated off by a partition from a working chamber enclosing the rotor. The working chamber together with the rotor defines at least one shear gap. A pump device, as disclosed in U.S. Pat. No. 3,208,041, delivers the shear fluid from the working chamber to the reservoir by way of a first opening provided in the partition in the region of the external diameter of the rotor. The supply of the shear fluid from the reservoir to the working chamber is controlled in dependence upon temperature by a valve. The torque transmitted by the fluid friction coupling depends upon the state of filling of the shear gap with shear fluid. In the cold condition the valve blocks the supply of shear fluid to the working chamber and the pump device pumps the shear fluid away, whereby the coupling is disengaged. In the hot condition the valve opens, whereby the shear gap is filled with shear fluid for the transmission of torque.
In conventional fluid friction couplings the shear fluid flows back by reason of the force of gravity through the opening of the pump device into the working chamber, when the coupling is not rotating. This is undesired since when the working chamber is filled the cold-start behavior deteriorates, especially at very low temperatures. Despite the fact that the internal combustion engine has cooled down, in such a case the fan coupling transmits the rotating movement of the drive shaft to the fan blades practically without slip, which is detrimental to a rapid warm-up of the engine. Furthermore the shear fluid is pumped away out of the working chamber only slowly, since the relative rotation rate between fan and housing is low. Moreover the fan coupling generates increased fan noises.
In European Patent Application No. 9,959 to improve the cold-start behavior the pump device opening provided in the radially outer region of the partition is conducted by way of a piece of tube in the reservoir close to the axis of rotation. This prevents the possibility of shear fluid running back into the working chamber when the fan coupling is not rotating. However the way of improving cold-starting as proposed in the European Patent Application brings only partial success, since it is a pre-requisite for this way that the fan coupling was disengaged and the shear fluid was situated substantially in the reservoir, when the internal combustion engine was halted. If however the engine is shut off with its cooling system in the hot condition, the fan coupling was engaged and the working chamber filled with shear fluid. On a subsequent cold start thus the above-explained disadvantages of conventional fan couplings result.
It is the problem of the invention to indicate a constructively simple way in which the cold-start behavior of the fluid friction coupling can be improved without it being important whether the engine is shut off in the cold or the hot condition.